Conventional refrigerating cycle apparatus each including a compressor, a radiator, a decompressor, a vaporizer connected to one another are used in air conditioners for buildings, air conditioners for vehicle use, electric (or freezers) refrigerators, refrigerating or freezing warehouses, showcases and the like. In such conventional apparatus, hydrocarbons containing fluorine atoms were used as a refrigerant.
Especially hydrocarbons containing both fluorine atoms and chlorine atoms (hydrochlorofluorocarbons (HCFCs)) were extensively used in refrigerating cycle apparatus because of their high performance, non-flammability and non-toxicity to human bodies.
However, it was revealed that HCFCs (hydrochlorofluorocarbons), because they contain chlorine atoms, would deplete the ozone layer when they were discharged into the atmosphere and eventually reached the stratosphere. Accordingly, HCFCs are being replaced by HFCs (hydrofluorocarbons), which contain no chlorine atoms. However, even though they have no property to deplete the ozone layer, they have a significant greenhouse effect on account of their long life in the atmosphere, and are not necessarily a satisfactory refrigerant for preventing global warming, which is a matter of grave concern nowadays.
In place of the aforementioned HCFCs and HFCs containing halogen atoms, the feasibility of refrigerating cycle apparatuses using carbon dioxide gas (CO2) as refrigerant, of which the ozone depletion factor is zero and the global warming factor is far smaller than that of hydrocarbons containing halogen atoms. For instance, in Japanese Patent Laid-Open No. 7-18602, there is proposed a refrigerating cycle apparatus using carbon dioxide gas (CO2)
The critical temperature of carbon dioxide gas (CO2) here is 31.1° C., and its critical pressure is 7372 kPa. In a refrigerating cycle apparatus using it, the cycle can be transcritical as will be described with reference to FIG. 11.
FIG. 11 is a Mollier diagram of the refrigerating cycle where carbon dioxide gas (CO2) is used as refrigerant. As indicated by A-B-C-D-A in the diagram, by depriving external fluid such as air of heat with the latent heat of vaporization, the external fluid is cooled by a compression stroke (A-B) of compressing CO2 in the gaseous phase state with a compressor, the cooling stroke (B-C) of cooling CO2 in the supercritical state of high temperature and high pressure with a radiator (gas cooler) the decompression stroke (C-D) of reducing its pressure with a decompressor, and the vaporization stroke (D-A) of vaporizing CO2 in the vapor-liquid two phase state with a vaporizer.
In FIG. 11, a line (B-C), positioned by the gas-liquid critical point CC toward the higher pressure side, crosses neither the saturated liquid curve nor the saturated steam curve. Thus, although the shift from the saturated steam region (the vapor-liquid two phase region) to the heated steam region (the gaseous phase region) in the vaporization stroke (D-A) takes place in a similar way to what takes place with HCFCs or HFCs, there is no condensation stroke, which is present with HCFCs or HFCs, in the region beyond the critical point CC (the supercritical region), but there comes the cooling stroke, in which CO2 is cooled without being liquefied.
As the working pressure of the refrigerating cycle apparatus using carbon dioxide gas (CO2) then is about 3.5 MPa on the lower pressure side and about 10 MPa on the higher pressure side, the working pressure is higher than where HCFCs or HFCs are used, the higher side pressure and the lower side pressure being about 5 to 10 times as high as in a refrigerating cycle apparatus using HCFCs or HFCs.
Next will be described the configuration of the refrigerating cycle apparatus. As the critical temperature of the refrigerating cycle apparatus using carbon dioxide gas (CO2) mentioned above is 31.1° C. and its critical pressure is 7372 kPa, its configuration is such that the high pressure side circuit (the refrigerant circuit from the discharge part of the compressor via the radiator to the inlet part of the decompressor) is used in the supercritical region, while the configuration of a usual refrigerating cycle apparatus is, as shown in FIG. 13, consists of a main route provided with a compressor 210 for raising the pressure of a refrigerant, a radiator 220 for cooling the refrigerant, a decompressor 230 for reducing the pressure of the refrigerant to its vaporization pressure and a vaporizer 240 for vaporizing and gasifying the refrigerant.
In this main route, the refrigerant, which has been raised in pressure by the compressor 210 and is in a supercritical state, is cooled by the radiator 220, reduced in pressure by the decompressor 230 to become wet steam and, after being turned into a gaseous phase by the vaporizer 240, is returned to the compressor 210.
If such a refrigerating cycle apparatus is configured in a similar way of thinking to the concept of the conventional refrigerating cycle apparatus using chlorofluorocarbon gas, where it is to be used for a vehicle-mounted air conditioner, it is a common practice to enclose about 500 g of carbon dioxide gas (CO2) as refrigerant and about 300 g of refrigerating machine oil as lubricating oil for the compressor 210. Thus, oil in a ratio of about 60 weight % of the enclosed carbon dioxide gas (CO2) is enclosed.
However, in a refrigerating cycle apparatus in which a large quantity of oil is enclosed similarly to such a refrigerating cycle apparatus using chlorofluorocarbon gas, the quantity of refrigerating machine oil discharged into the cycle is also large, and where carbon dioxide gas (CO2) is used for which the high pressure side circuit (the refrigerant circuit from the discharge part of the compressor via the radiator to the inlet part of the decompressor) is used in the supercritical region and there is no condensate, the refrigerating machine oil discharged into the cycle may stick to the inner walls of the pipes of the radiator 220 or become scattered in a misty form, inviting inhibition of heat transfer or an increase in pressure loss and thereby becoming a cause of an enlargement of the size or a drop in the efficiency of that refrigerating cycle apparatus.
On the other hand, as the refrigerant channels of heat exchangers used in the radiator and the vaporizer of a refrigerating cycle apparatus using carbon dioxide gas (CO2) as refrigerant, a flat tube 21 configured of a plurality of small bore through-holes 21a is employed as shown in the schematic configurational diagram of FIG. 12 to bear the pressure of the high pressure refrigerant.
Here, when oil is discharged together with CO2 from the compressor, the oil will become a factor to inhibit the vaporization of the CO2 in the refrigerant channels, especially in the refrigerant channels in the vaporizer having a plurality of small bore through-holes, inviting inhibition of heat transfer or an increase in pressure loss, and accordingly there has been found the problem that the size of the refrigerating cycle apparatus is increased or its efficiency deteriorated.